From the old Fordbarn, a gent suggested to me that he used Black Satin from Speedway in a V-8 60 that he ran in a midget. He did not know how much it helped because there was no before and after analysis. I gave it a try on my 8BA that is in my '36 pickup, but I only applied it to the center port. Soon I will be building some new headers for my truck and when I remove the old ones I guess I'll be able to see if it stuck OK.
I prepped the center port by putting some tape over the seats to protect them. Then I installed a couple of old valve assemblies. I sealed off the heat riser holes. Now with one of those spot sand blaster guns that have the bags on them to recycle the media, I put the nose of that up the exhaust port to blast it. I hoped that I was creating a mini blasting cabinet inside of the exhaust port and valve cavity. Following clean up from that, I used an airbrush gun to shoot the Black Satin into the port and cavity from below and above.
I always thought that I could measure the effectiveness of the coating by using an infared temp gage. Alas, the friend I had with one of those is not available, so I have not done that. My truck runs well and I control the temp OK but I have a lot of mods on it, the exhaust dividers, the 409 water pump and a pusher electric fan that I think sometimes impedes air. So, there is no real telling how well the coating works.